I've been messing about a bit to try and get my debugger (VS2010) to allow me to step into the framework source code. (msdn) This didn't work and I ran out of time so changed my settings back, and now I don't get dropped into the debugger when a web project (in debug mode) throws an error, I simply get the yellow screen of death.
What's strange is that I can put a breakpoint in my code which works fine, and I can step over my code but as soon as I hit the line causing the error it drops out of the debugger back to the error in the browser.
I'm sure I've just got a setting wrong somewhere but its incredibly frustrating!
A colleague saved me! This article about Break when exception unhandled had the solution, I had the "Enable just my code" unchecked in Tools > Options > Debugging. No idea how it happened, but fixed now!